This invention relates to the direct conversion of methane and acetylene into isobutene.
Isobutene is typically recovered as a product from refinery light-end operations by conventional methods. The isobutene is used primarily as a reactant together with formaldehyde to synthesize isoprene. Isobutene may also be reacted with methanol to produce methyl tertiary butyl ether (MTBE). Other possible uses for isobutene include hydration to gasoline grade tertiary butyl alcohol, alkylation with C.sub.4 olefins to make alkylation gasoline, polymerization to diesel fuel, jet fuel and polymer gasoline and as an RBP blending component.
Demands for isobutene are expected to increase dramatically and this has necessitated exploring new synthesis routes for the alkene. However, new sources of isobutene have been limited to dehydrogenation of isobutane from field butanes.
The reaction of methane with acetylene under mild conditions in the presence of Fe(acetylacetonate).sub.3 -Et.sub.3 Al catalyst has been reported to lead to the formation of propylene with complete suppression of the self-polymerization of acetylene (Grigoryan, E.A. et al. Dokl. Akad. Nauk SSSR, 257, 364, 1981). The formation of propylene is believed to occur via insertion of acetylene at a Fe-CH.sub.3 bond and subsequent elimination of a metal hydride complex of Fe.
The reaction between methane and acetylene to C.sub.4 hydrocarbons is thermodynamically favorable over a range of temperatures as shown by the free energy calculations in the following table:
______________________________________ Free energy for some possible reactions .DELTA.G (kcal/mol) Reaction 300 K 400 K 500 K ______________________________________ 1. 2CH.sub.4 + C.sub.2 H.sub.2 .fwdarw. -9 -5 -2 C.sub.4 H.sub.8 + H.sub.2 2. 2CH.sub.4 + C.sub.2 H.sub.2 .fwdarw. -30 -24 -17 C.sub.4 H.sub.10 3. 2C.sub.2 H.sub.2 + 3H.sub.2 .fwdarw. -104 -92 -80 C.sub.4 H.sub.10 4. 4CH.sub.4 .fwdarw.C.sub.4 H.sub.10 + 3H.sub.2 44 45 46 ______________________________________ .DELTA.G.sub.rxn = .DELTA.G.sub.pro - .DELTA.G.sub.rea
Alkylations such as that of isobutane with isobutene or any other C.sub.4 olefin to produce C.sub.8 alkylate are well known in the petrochemical industry. These alkylations are acid catalyzed and the catalysts employed are HF and H.sub.2 SO.sub.4. Until recently, the alkylation of the lower alkanes with alkenes has been studied in liquid acid media and liquid phase reaction conditions were considered to substantially limit practical application of the reaction. In addition, the products were a mixture of oily oligomers whose molecular weight ranged from 100 to 700.
The alkylation of methane and olefins (alkynes) has received special attention since it was reported in Olah, U.S. Pat. No. 4,465,893, issued Aug. 14, 1984, that alkylation between methane and ethylene could be conducted over a solid superacid catalyst.
It is the object of the present invention to find a simple method for producing isobutene starting from methane or natural gas as a reactant.